One or more layers of refractory material are often applied to cutting tool surfaces by physical vapor deposition (PVD) techniques to increase properties including wear resistance, performance and lifetime of the cutting tool. Titanium nitride (TiN) coatings, for example, are commonly applied by PVD to cemented carbide cutting tool substrates. However, TiN begins oxidation at about 500° C. forming rutile TiO2, thereby promoting rapid coating deterioration. Incorporation of aluminum into the cubic lattice can slow degradative oxidation of a TiN coating by forming a protective aluminum-rich oxide film at the coating surface.
While providing enhancement to high temperature stability, aluminum can also induce structural changes in a TiN coating having a negative impact on coating performance. Increasing amounts of aluminum incorporated into a TiN coating can induce growth of hexagonal close packed (hcp) aluminum nitride (AlN) phase, altering the crystalline structure of the coating from single phase cubic to a mixture of cubic and hexagonal phases. Aluminum content in excess of 70 atomic percent can further alter the crystalline structure of the AlTiN layer to single phase hcp. Significant amounts of hexagonal phase can lead to a considerable reduction in hardness of AlTiN, resulting in premature coating failure or other undesirable performance characteristics. The inability to sufficiently control hexagonal phase formation has obstructed full realization of the advantages offered by aluminum additions to TiN coatings.
Further, PVD coatings, including AlTiN, are limited in thickness due to high residual compressive stresses induced by ion bombardment during the deposition process. Residual compressive stress only increases with coating thickness rendering the coating susceptible to delamination or other adhesive failure mode. Bias voltage of the substrate can be reduced to mitigate residual compressive stress in PVD coatings. Nevertheless, reduction in bias voltages can significantly compromise coating hardness. For example, in AlTiN and similar systems, reduction in bias voltage promotes hexagonal phase formation.
In view of these considerations, significant barriers exist to providing PVD coatings of high hardness, high thickness and low residual compressive stress.